The field of sonar (sound navigation and ranging) relates to the detection of targets in a body of water by reflecting sound waves off targets and processing the return signals. Many systems in the frequency range of 1 to 60 Khz have been developed for such detection.
High frequency sonars generate an image by "taking a picture with sound" and can be grouped into sidelooking and forward looking geometries. In sidelooking sonar, an underwater carrier vehicle towed by a surface vessel travels at a certain altitude above a target area such as the sea bottom. Sidelooking sonar apparatus on the carrier vehicle is operative to repetitively transmit acoustic pulses to the side of the carrier vehicle with two such pulses, a starboard pulse and a port pulse. The pulses ensonified areas in the form of strips.
Acoustic returns from elemental areas during their course of travel, are received by the sidelooking sonar apparatus which is then operable to form one or more receiver beams for examining the ensonified strips. The received acoustic energy is processed and displayed on a suitable display apparatus. With each pulse transmission and subsequent reception, a scan line is produced on the display to build up a picture of the sea bottom in a manner similar to the scanning of a conventional cathode ray beam in a television picture tube with the presentation being a pattern of highlights and shadows with objects outlined in such a way as to permit their identification.
However, high speed multibeam sidelooking sonar systems have two problems which affect their close-in performance:
(1) receiver grating sidelobes overlapping the transmit beam, and
(2) excessive projector fill time degradation. Both of these problems have been solved by the device of the present application.